【作者】 韩靖；

【导师】 盛光敏；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2009， 博士

【摘要】 在异种金属材料的固态扩散连接过程中,为了实现原子的互扩散和有效的冶金结合,需在较高温度下进行长时间连接。但由于异种金属材料之间存在较大的物理、化学和力学性能差异,在连接界面处易生成较厚的金属间化合物,扩散层晶粒粗化,接头内应力增大,从而恶化了接头的力学性能。针对这一困扰工程界多年的问题,本文采用高能喷丸（High energy shot peening, HESP）工艺,对0Cr18Ni9Ti不锈钢与TA17近α钛合金的连接界面进行表面自纳米化（Surface self-nanocrystallization,SSNC）处理,获得了一定厚度的纳米晶组织,以期望促进原子在扩散连接过程中的互扩散,降低连接温度,缩短连接时间,抑制脆性金属间化合物的生长,改善接头组织,提高接头力学性能。这由于表面自纳米化纳米组织中含有高体积分数的晶界,能够为原子扩散提供大量通道,同时纳米组织中具有较高的吉布斯自由能,降低了原子的扩散激活能,有利于提高扩散连接过程中原子的扩散系数。文中对高能喷丸处理后的不锈钢和钛合金的纳米组织进行了表征;试验研究了不锈钢和钛合金表面层纳米组织的热稳定性;采用恒温恒压、脉冲加压对表面自纳米化0Cr18Ni9Ti不锈钢与TA17近α钛合金进行扩散连接;借助拉伸实验机、金相观察、显微硬度测试、扫描电镜观察、能谱分析、X-射线衍射等分析手段研究了表面自纳米化钛合金与不锈钢扩散连接接头的组织结构和性能。研究表明,采用高能喷丸对0Cr18Ni9Ti不锈钢与TA17钛合金进行表面自纳米化处理后,分别在端面以下70μm和50μm左右深度内形成了等轴纳米晶。晶粒细化机理与晶格结构和层错能密切相关,不锈钢的晶粒细化机理为位错+孪生变形+应变诱导马氏体相变机制;钛合金的粒细化机理为孪生+位错滑移机制。在高能喷丸过程中,应力和应变速率随着深度的增加逐渐减小,从而使变形层内的晶粒尺寸随深度增加逐渐增大并过渡到基体晶粒尺寸。采用退火处理对钛合金与不锈钢的高能喷丸纳米组织进行了热稳定性试验研究,结果表明不锈钢与钛合金的高能喷丸纳米晶组织分别在不超过650℃和550℃退火时,有相对较好的热稳定性;不锈钢与钛合金纳米组织在850℃退火30min后,晶粒尺寸分别不超过80nm和100nm。在800～900℃之间,采用8MPa恒定轴向压力,对表面自纳米化0Cr18Ni9Ti不锈钢和TA17钛合金进行恒温恒压扩散连接（Constant Temperature and Pressure Diffusion Bonding, CTPDB） 20min后,在850℃连接时获得了最高拉伸强度为327MPa接头,比粗晶试样采用相同扩散连接工艺获得的接头拉伸强度高出60MPa以上。在650～750℃之间对表面自纳米化0Cr18Ni9Ti不锈钢和TA17钛合金采用脉冲加压扩散连接（Pulse Pressure Diffusion Bonding, PPDB）后,获得了具有一定拉伸强度的连接接头。本文首次在低于800℃以下获得了钛合金与不锈钢的扩散连接接头。经对800℃以下的脉冲加压扩散连接工艺参数进行优化,在升温和降温速度一定,压力脉冲频率一定的条件下,得到优化工艺参数为:连接温度T=750℃,最大脉冲压力Pmax=150MPa,最小脉冲压力P_min=80MPa,脉冲次数n=400次,在此工艺下得到的接头拉伸强度达到262MPa。在850℃时,采用最小脉冲压力Pmin=8MPa,最大脉冲压力Pmax=50MPa,脉冲频率f=0.5Hz的工艺参数,对表面自纳米化TA17钛合金和0Cr18Ni9Ti不锈钢进行脉冲加压扩散连接80s后,获得了最大拉伸强度为384.0MPa的接头,比粗晶试样采用相同扩散连接工艺获得的接头拉伸强度高出60MPa以上。对上述不同扩散连接工艺下得到的接头的物相和组织进行分析研究,结果表明,从不锈钢侧到钛合金侧依次形成了不同的物相,分别为奥氏体（不锈钢基体）、金属间化合物、β-Ti固溶体和α-Ti固溶体（钛合金基体）。金属间化合物中主要有σ相、Fe₂Ti、FeTi等,金属间化合物层的厚度随连接温度的降低而减少。与常规粗晶试样相比,经相同扩散连接工艺连接后,表面自纳米化试样接头中金属间化合物的数量减少,分布得到了改善,从而提高了接头的力学性能。对接头拉伸断口分析表明,断裂发生在金属间化合物与β-Ti固溶体交界区域,拉伸时β-Ti固溶体承受了主要拉伸载荷,金属间化合物是导致裂纹扩展和断裂的主要原因。对表面自纳米化试样中原子扩散动力学进行了计算,计算结果与常规粗晶试样中对应的结果对比表明,Fe、Ti原子的扩散激活能降低,扩散系数提高。通过本试验研究表明,将表面自纳米化技术应用于钛合金与不锈钢的扩散连接中,实现了钛合金与不锈钢的低温、短时、高效连接,改善了接头组织,提高了接头性能,最大限度地减小了金属间化合物的有害作用,为异种金属的连接提供了一种新的方法。更多还原

【Abstract】 In order to effective metallurgical bonding of dissimilar metal, diffusion bonding is processed at high temperature for long time. Resultly, thicker intermetallic compounds is yielded easily on the bonding interface, crystal grains grow significantly in the diffusion layer, and innerstress is large in the joint because of tremendous differences of physical, chemical and mechanical property, which deteriorated badly mechanical character of joint. For the sake of putting the axe in the helve, in the paper, surface self-nanocrystallization (SSNC) was apllied to synthesize nanostructured layer on the bonded surfaces of 0Cr18Ni9Ti stainless steel and TA17 titanium alloy bars by means of high energy shot peening (HESP), which aims at increasing atomic diffusion coefficient, reducing bonding temperature and restraining of brittle intermetallic compounds at bonding surface, betterment of microstructure in the diffusion layer, advancing of tensile strength. Because a large volume fraction of grain boundaries in nanostructure may act as fast atomic diffusion channels, and large number of nonequilibrium defects with high stored energy may reduce activation energy, which may increase atomic diffusion coefficient while diffusion bonding.After samples SSNCed, microstructures were characterized in the deformation layer, and thermal stability of nanostructure was put on test. Constant temperature and pressure diffusion bonding (CTPDB) and pressure and pulse pressure diffusion bonding (PPDB) were applied to produce SSNCed 0Cr18Ni9Ti stainless steel/TA17 titanium alloy joints. Microstructure observation, micro-hardness testing, scanning electron microscope (SEM) observation, energy dispersive spectroscope (EDS) and X-ray diffraction (XRD) analysis were employed to investigate the structure and performance of the joints.After SSNCed for 5min by means of HESP, surface nanostructured layer about 70μm and 50μm thickness were obtained on the stainless steel and titanium alloy ends respectively. Refinement mechanism of coarse grains for metal depend strongly on the lattice structure and the stacking fault energy while SSNCed. For 0Cr18Ni9Ti stainless steel, refinement mechanism is dislocation slip and twinning deformation and strain-induced martensitic transformation, and twinning deformation and dislocation slip for the titanium alloy. Stress and strain rate is reducing with increasing of depth to top surface, resulting that size of grains is increasing in the deformation layer and transition to that of matrix gradually.Surface nano-microstructures keep good thermal stability in the stainless steel and titanium alloy when annealing temperature is no more than 550℃and 650℃respectively. Furthermore, nano grains size of stainless steel and titanium alloy don’t exceed 80nm and 100nm respectively even if annealed at 850℃for 30min.CTPDB was applied to produce joints of SSNCed TA17/0Cr18Ni9Ti in the temperature rang of 800～900℃under a uniaxial load of 8MPa in vacuum for 20min, and effective joints were formed. When bonding temperature was 850℃, the maximum tensile strength of joint was as high as 327MPa. At the same diffusion bonding technology, the value is higher above 60MPa than that of their conventional coarsegrained bonded joint.PPDB was applied to prepare joints of SSNCed TA17/0Cr18Ni9Ti in the temperature rang of 650～750℃in vacuum, and joints with certain tensile strength were achieved. For the first time, joint of titanium alloy/stainless steel was obtained at below 800℃in this paper. While heating rate before pulse and cooling velocity after pulse were 5℃/s, pulse frequency was 0.5Hz, the optimized parameters for PPDB below 800℃were gained as following: bonding temperature was 750℃, pulse pressure was 80～150MPa, pulse pressuring times was 400 cycles. The joint strength under the optimum condition was 262.0MPa.By means of PPDB, SSNCed TA17/0Cr18Ni9Ti was bonded at 850℃for 80s. The pulse pressure is 8～50MPa, pulse frequency is 0.5Hz and the cycles is 40times. The joint tensile strength of 384.0MPa was achieved. At the same diffusion bonding technology, the value also is higher above 60MPa than that of their conventional coarsegrained bonded joint.Microstructures of joints by mentioned above diffusion bonding technology were researched. The results showed that multi-microstructures, which are in turnγ-Fe, brittle intermetallic compounds,β-Ti andα-Ti from stainless steel side to titanium alloy side, were formed on the joint. The intermetallic compounds are mainly FeTi、Fe2Ti andσphases. Thickness of compounds is decreasing with reducing of bonding temperature. Relativing to conventional coarse-grained joints, the thickness of compounds in SSNCed joints are thinner at the same bonding technology. Research of joint fractures showed theβ-Ti beared principal tensile load while joints were tensile test, and the brittle intermetallic compound is the prime reason of fracture. Diffusion coefficient and activation energy of atoms in the diffusion layer were calculated, the results showed that the activation energy is far lower than that in coarse-grained samples, and the diffusion coefficients is larger than that in coarsegrained samples.In the paper, research results showed that application SSNC to diffusion bonding of titanium alloy and stainless steel decreased activation energy of diffusion atoms significantly, increased diffusion coefficient atoms, suppressing remarkably growth of intermetallic compounds, improved microstructures of joints, increased tensile strength of joints. With SSNC treatment before bonding, the temperature and time of diffusion bond is lowered, the performance of joint of is improved, and the negative effects of intermetallic compounds for mechanical behaviour of joint was reduced as possible as. Through this research, an improved technology for the bonding of dissimilar materials was developed.更多还原